Automotive batteries, typically termed SLI automotive batteries, principally used for starting, lighting and ignition requirements of an automobile, have commonly used lead-acid technology.
Over the last 20 or so years, there has been substantial interest in automotive lead-acid batteries which require, once in service, little, or more desirably, no further maintenance throughout the expected life of the battery. This type of battery is usually termed a "low maintenance" or "maintenance-free" battery. Such low maintenance or maintenance-free batteries were first commercially introduced in about 1972 and are currently in widespread use.
It has been well recognized over the years that lead-acid batteries are perishable products. Eventually, such batteries in service will fail through one or more of several failure modes. Among these failure modes are failure due to positive grid corrosion and excessive water loss. The thrust of maintenance-free batteries has been to provide a battery that would forestall the failure during service for a period considered commensurate with the expected service life of the battery, e.g., three to five years or so.
To achieve this objective, the positive grids used initially for maintenance-free batteries typically had thickness of about 60 to 70 mils or so. The batteries were likewise configured to provide an excess of electrolyte over that needed to provide the rated capacity of the battery. In that fashion, by filling the electrolyte to a level above that of the top of the battery plates, maintenance-free batteries contained, in effect, a reservoir of electrolyte available to replenish the water loss during the service life of the battery. In other words, while the use of appropriate grid alloys will reduce water loss during the service life of the battery, there will always be some loss in service. Having an excess of electrolyte by design will compensate for this loss.
One complicating factor in attempting to provide satisfactory service life is the seemingly ever-increasing power and energy requirements demanded in current SLI automotive batteries used in modern automobiles. Many factors have contributed to the need and/or desire for such higher power and energy for such batteries. One major measure of power currently in common usage is the rated number of cold cranking amps. The number of cold cranking amps is considered in the industry as some indication of the relative power of the battery to start an automobile in cold temperature conditions.
Yet another complicating factor is the "under-the-hood" space requirements. Automobile manufacturers have significantly reduced the overall space available for batteries in the engine compartment. Typically, this has required that battery manufacturers provide a lower profile battery, viz., a battery having less overall height than previously required so as to meet current aerodynamic styling needs in automobiles. Such lower profile batteries will have less acid above the plates.
Another aspect that has occurred in recent years is the substantial increase in the under-the-hood temperature to which the battery is exposed in automobile service. Obviously, the under-the-hood temperature is particularly high in the warmer climates. One automobile manufacturer has perceived that in the past few years, the temperature to which an SLI battery is exposed under-the-hood in such warmer climates has risen about 125.degree. to about 165.degree. to 190.degree. F. in new automobiles.
Specific temperature increase which is involved is not particularly important. What is important is that such under-the-hood temperatures have, in fact, increased. The impact of this increase in the under-the-hood vehicle service temperatures on the failure modes has been to substantially increase the occurrence of premature battery failures. The incidents of premature battery failures due to excessive positive grid corrosion has been significant.
Probably the most widely used technique still for making SLI battery grids has been the conventional book mold gravity casting technique. It has, however, long been recognized that this technique, semi-continuous at best, can cause several production problems. In the first place, gravity casting techniques are subject to various problems which result in scrap as well as lack of product consistency and the like. These problems include operator errors; wide variation in grid wire thickness and hence overall weight due to mold coating variations and irregularities; substantial material handling in production and difficulty in automating such processes and the accompanying inconsistencies due to human error and the like.
Feeding of these individual grid panels by gravity casting techniques into the pasting machine during high speed production conditions can also result in frequent grid jam ups and with resultant scrap. Further, such jam ups result in production stoppage, lost production, clean-up of jams and variation in paste machine set-up and attendant paste weight and paste thickness variations.
Another problem of substantial significance stems from the environmental issues involved in pasting, curing, and assembly of batteries using gravity cast SLI battery grids. Lead dust is a major problem, stemming from loss of powdery active material from cured and dried paste during processing and handling while assembling batteries. Mechanical handling loosens powdery active material since there are no surface barriers. The resulting lead dust must be dealt with in an environmentally satisfactory manner, and production staff have to wear respirators while carrying out pasting and battery assembly operations. Indeed, a great many production safeguards need to be provided to handle powdery lead oxide dust.
Potentially, the use of any continuous process like continuous grid casting or other continuous expanded metal fabrication techniques to make battery grids is capable of minimizing, if not eliminating, one or more of the problems associated with gravity casting techniques. There has accordingly been substantial interest and effort directed to the use of such techniques over the years. This effort has resulted in what is believed to be the widespread use of various continuous, expanded metal fabrication process for making SLI negative battery grids.
The same benefits would result when using continuous processing for making grids and plates for SLI positive battery grids. However, one major issue is present with positive grids and plates that is not an issue with negative battery grids and plates. More particularly, as has been previously alluded to herein, corrosion of the positive battery grid is a principal mode of failure of SLI batteries. At least for this reason, as far as has been perceived, expanded metal fabrication techniques had not been widely used commercially for making SLI positive battery grids, because of increased susceptibility of the continuous cast strip which is expanded into SLI positive grids to positive grid corrosion, prior to the invention set forth in U.S. Pat. No. 5,434,025 to Rao et al.
Various continuous processes for making grids from cast and rolled strip have been proposed. One particularly desirable approach utilizes a directly cast strip, i.e., a continuous strip that is directly cast from molten lead alloy into the thickness desired for making the grids. The casting process thus does not include any cold rolling or other reduction in the thickness of the strip from the cast thickness to the thickness desired for making the grid. Equipment for making a suitable directly cast alloy continuous strip from molten lead alloy is commercially available (Cominco Ltd., Toronto, Canada). In this regard, U.S. Pat. No. 4,315,357 to Laurie et al. illustrates, in general, the method and apparatus for forming the expanded mesh strip necessary for making a continuously cast grid.
U.S. Pat. No. 5,384,217 to Binder et al. refers to problems which can arise in the manufacture of plates from a moving strip or the like. More particularly, it is stated that, in one common battery design, every other plate in the battery stack is inserted into an envelope made of a separator material. The sides of the envelope act as separators between the plates and envelope the two adjoining plates in the battery stack. In assembling a battery of this kind, Binder et al. state that it is necessary to insert the battery plate bottom-first into the open end of the envelope so that the conductive tab at the top of the plate extends out of the envelope. However, the bottom corners on the battery plate are sharp, and will snag and tear the separator material between the positive and negative plates, causing an electrical short within the battery and reducing battery life. It is also stated that bending or vibration of the plate disposed in the envelope during assembly or use can cause tearing, and the problem is not confined to envelope-style separators. Use of battery plates with rounded bottom corners would eliminate the snag and tearing of separators, but, it is stated, no practical process has been proposed for producing rounded corners on such battery plates. In particular, it was noted that any process wherein the rotary divider (i.e., the apparatus described which forms the individual plates by cutting the outline of the individual plates on the moving strip) cuts off corners and results in small pieces of trim (scrap) that are severed from the strip would lead to battery failure if the pieces, if left on the strip after cutting, came loose. The '217 patent describes a process and apparatus for forming a plate by transporting the strip past a divider having a rotary cutter using a plurality of cutting blades, which include lengthwise blades (forming the sides of each plate), central transverse blades (forming the tab on each plate), and rounded blades (for forming the rounded lower corners of each plate). As the blades cut the strip to form the plates and pieces, a vacuum system applies suction to draw the pieces cut from the strip inwardly into the cutter through holes in the cutter, and then out of the cutter.
The approach described in the '217 patent is not considered to provide a satisfactory solution to the diverse problems resulting from making grids in a continuous fashion. In the first place, the '217 patent only discloses carrying out the corner-rounding process at the plate dividing step in the manufacturing of the grid, and, thus, requires extremely complicated processing and apparatus because the moving strip already has been pasted. Secondly, the requirement that the bottom corners be rounded also complicates the apparatus required. Moreover, and contrary to the '217 patent, bottom corner rounding will not eliminate much of the potentiality for separator damage resulting from the configuration of the grid. Still other concerns regarding the '217 approach are that the blades involved are relatively fragile and repair would be costly. Also, such an accommodating structure as envisioned in the '217 patent might possibly weaken the structural integrity of the die.
Thus, there exists a need for a facile process that allows continuous manufacture of lead-acid battery grids so as to obtain the substantial benefits that can be derived from continuous processing, yet which satisfactorily addresses the diverse requirements for such processing.
It is accordingly a principal object of the present invention to provide a commercially viable process for making battery grids using continuous cast grid manufacturing methods.
Another object provides a method, and the resulting grid, which at least minimizes, if not eliminates, potential problems resulting from grid configurations that could weaken, or even tear, the separators used in the battery.
Yet another object lies in the provision of a method for making battery grids which facilitates the assembly of the battery itself made using grids manufactured according to the present invention.
A still further object of the present invention is to provide a process capable of being reliably run at commercial rates of speed with minimal scrap rates due to separator punctures and the like.
Another object of the preferred method of this invention includes a process allowing freedom from defects due to separator impairment, even when the process is operated in less than an optimum fashion. Stated differently, an object of this invention provides a process having wide manufacturing tolerances.
Other objects and advantages of the present invention will become apparent as the following description proceeds.